Positioning servosystems and like apparatus frequently will take advantage of dual mode operating circuitry. In a "seeking" mode, high power at maximum speed is of the essence, while in a "following" mode, low power with low electric noise generation is crucial.
The class A amplifier circuit is adaptable to the "following" mode in spite of poor efficiency because of its low noise. The Class D amplifier circuit has excellent efficiency physically for digital and similar applications and is capable of handling relatively high powr without a severe loss in efficiency.